Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another to form the cylinders. To control the charge exchange, an internal combustion engine requires control elements—generally in the form of valves—and actuating devices for actuating said control elements. The valve actuating mechanism required for the movement of the valves, including the valves themselves, is referred to as the valve drive. The cylinder head often serves to accommodate the valve drive.
During the charge exchange, the combustion gases are discharged via the outlet openings of the cylinders, and the charging of the combustion chambers, that is to say the induction of the combustion air, takes place via the inlet openings. If the internal combustion engine is equipped with an exhaust-gas recirculation system, the combustion air may also contain exhaust gas in addition to the fresh air sucked in from the environment. If the fuel is not injected directly into the cylinders but rather is introduced for example into the intake tract upstream of the cylinders, not only the combustion air but rather also the fuel is supplied to the cylinders via inlet openings.
It is the object of the valve drive to open and close the inlet and outlet openings at the correct times, with a fast opening of the largest possible flow cross sections being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure the best possible charging of the cylinders with fresh mixture, and an effective, that is to say complete, discharge of the exhaust gases.
According to previous approaches, the intake lines which lead to the inlet openings are at least partially integrated in the cylinder head and are generally merged, often to form a single overall intake line, such that at least one so-called inlet manifold is formed.
Various demands are placed on the inlet region of an internal combustion engine. It is sought inter alia to provide an arrangement and design of the intake lines which leads to as small as possible a pressure loss in the intake combustion air in order to ensure good charging of the cylinders with fresh mixture.
The geometry of an intake line furthermore has an influence of the charge movement in the cylinder and therefore on the mixture formation, in particular in direct-injection internal combustion engines. The intake lines are therefore often designed so as to generate a so-called tumble or a spinning flow which should accelerate and assist the mixture formation, wherein a tumble is an air swirl about an imaginary axis which runs parallel to the longitudinal axis of the crankshaft, and a spin is an air swirl whose axis runs parallel to the piston or cylinder longitudinal axis.
During the charge exchange, the pressure along the flow path in the intake duct varies. Such local pressure fluctuations propagate as waves in gaseous media. To make it possible to utilize these dynamic wave processes for the optimization of the charge exchange, it is for example possible for the inlet region to be designed such that, toward the end of the intake stroke, a positive pressure wave arrives at the inlet openings, which positive pressure wave leads to a compression and therefore to a certain follow-up charging effect. Intake lines of variable length are expedient here.
A multiplicity of additional lines, for example the recirculation line of an exhaust-gas recirculation system or the bypass line of a charge-air cooler or of a compressor, may open into the intake line or overall intake line.
Furthermore, internal combustion engines may be equipped with a heating device which is arranged in the inlet region, that is to say intake region, and which serves for heating the intake air. A heating device suitable for use in internal combustion engines is described for example in the German laid-open specification DE 102 14 166 A1 and in the European patent EP 0 793 399 B1.
Said heating devices known from previous approaches comprise strip-like heating elements which are electrically heatable and which have a rectangular cross-sectional outline. The strip-like heating elements are arranged in the inlet region in such a way that their rectangular cross section poses the least possible resistance to the intake combustion air. A first narrow side of the cross section of the strip-like heating elements faces toward the intake combustion air flow, whereas the long sides of the rectangular cross section extend in the flow direction, such that the intake combustion air flows tangentially over the larger longitudinal sides. Such an alignment of the cross section is expedient from a flow aspect but also advantageous with regard to the heat transfer by convection.
According to DE 198 54 077 A1 the heating device may basically also be arranged downstream of a charge-air cooler optionally provided in the intake region. The above-cited approach also discusses the construction of the heating device, in particular the flange which serves as a receptacle or frame, and the design of the strip-like heating elements and the materials or material mixtures used for these.
The heating of the intake air may serve various purposes, for example to shorten the warm-up phase after a cold start, as described in DE 198 54 077 A1.
The German laid-open specification DE 10 2006 030 464 A1 utilizes the heating of the intake air in large-volume diesel engines also outside the starting and warm-up phase in order to avoid misfires at idle when using fuels with a low cetane number. Furthermore, the heating element is activated during the regeneration of the particle filter, and also when the engine torque and the engine speed fall below a predefined minimum value.
However, the inventors herein have recognized that operating an intake heater may be beneficial during additional engine operating conditions. Thus, embodiments for operating an internal combustion engine are provided. One example method for operating an internal combustion engine having at least one cylinder head and at least two cylinders, in which each cylinder has at least one inlet opening for the supply of combustion air into the cylinder, comprises activating a heating device for heating the combustion air when a fuel supply of the internal combustion engine is deactivated, the heating device arranged in an overall intake line coupled to the internal combustion engine, the overall intake line including at least two merged intake lines, each intake line leading to an inlet opening of a respective cylinder.
If the fuel supply is deactivated, the release of heat during the combustion as a result of the exothermic chemical conversion of the fuel, which heat serves to keep the internal combustion engine at operating temperature, is eliminated.
It is therefore advantageous for the heating device to be activated so as to heat the combustion air when the fuel supply of the internal combustion engine is deactivated. This leads to an improvement in emissions behavior and to a reduction in fuel consumption.
It may be taken into consideration in this context that heating of the internal combustion engine by preheated intake air serves not only to maintain the operating temperature but also to maintain the temperature of the engine oil. The associated low viscosity of the oil results in low friction and friction losses, in particular in the bearings which are supplied with oil. This effect has an advantageous influence on the fuel consumption.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.